Field of the Invention
The present invention relates to grease, a rolling bearing, a rolling bearing device, and an information recording and reproducing apparatus.
Priority is claimed on Japanese Patent Application No. 2015-009485, filed on Jan. 21, 2015, and Japanese Patent Application No. 2015-236921, filed on Dec. 3, 2015, the content of which is incorporated herein by reference.
Description of Related Art
As an apparatus that magnetically or optically records various kinds of information on a disc and reproduces the information, an information recording and reproducing apparatus such as a hard disk drive (HDD) is known. Typically, the information recording and reproducing apparatus includes a swing arm in which a head gimbal assembly (magnetic head) that records a signal on a disc and reproduces the signal is provided at a tip end, a rolling bearing device that becomes a rotation supporting point of the swing arm, and an actuator that rotates the swing arm. When the magnetic head is moved to a predetermined position on the disc by rotating the swing arm, the recording and reproduction of the signal can be performed.
Typically, the rolling bearing device includes two rolling bearings in which a plurality of spherical rolling bodies are provided between an inner ring and an outer ring, and a shaft that is inserted to an inner side of the rolling bearing. The outer ring rotates around an axis of the shaft due to rolling of the plurality of rolling bodies, and the swing arm that is connected to the outer ring rotates along with the rotation. The rolling bearing is required to stably operate over a long period of time. Accordingly, grease is used to make movement of the rolling bodies between the inner ring and the outer ring smooth.
The grease for the rolling bearing of the information recording and reproducing apparatus is required to lower the torque on the rolling bearing, to obtain excellent torque smoothness (a property in which the torque is constant in a rotation direction of the rolling bearings), and to enhance durability of the rolling bearing. In addition, since outgas from the grease collects in a gap between the magnetic head and the disc, a problem relating to reading and writing occurs in the information recording and reproducing apparatus. Accordingly, it is important that the amount of outgas from the grease for the rolling bearing is small.
As the grease for the rolling bearing of the information recording and reproducing apparatus, for example, there is known grease that contains a base oil containing a mineral oil and poly-α-olefin (hereinafter, referred to as “PAO”), a thickener (a urea compound and the like), and an extreme pressure agent (an organic phosphorous compound and the like) (refer to Patent Document 1). In this grease, the amount of outgas is relatively small, and the grease causes there to be a low torque on the rolling bearing and the rolling bearing to have excellent torque smoothness and durability. However, recently, along with an increase in density of HDDs or an increase in a demand for server use, a distance between the disc and the magnetic head has reached a nano-order. Accordingly, there is a demand for an additional decrease in the amount of outgas and an additional improvement in durability.
As grease capable of reducing the amount of outgas, there is suggested grease for a rolling bearing in which only PAO is used as the base oil instead of using a mineral oil in which the amount of outgas is larger in comparison to PAO (refer to Patent Document 2). However, in the grease, the amount of outgas is small, but it is difficult to obtains a rolling bearing having sufficient durability.